122 ANALYSIS OF THE FOUR PRINCIPLBS (CONTINUED). 



the second case it is primarily separative, but inevitably passes into 

 segregate breeding. Divergence through diversity of use, and the 

 resulting diversity of acquired characters, whether inherited or not, 

 will operate as surely in the one case as in the other. 



Sustentational isolation arises from the use of different methods of 

 obtaining sustentation by members of the same species. 



There can be no doubt that of the innumerable cases where phyto- 

 phagic varieties (as they are sometimes called) of insects exist, a con- 

 siderable proportion would be found on investigation to be permanent 

 varieties, producing offspring that are better adapted to the use of 

 the special form of food consumed by the parents than are offspring 

 of other varieties ; and it is evident that if the peculiar habits of each 

 variety had no tendency to produce segregative breeding this result 

 would not be reached, for each variety would be promiscuously min- 

 gled with every other, and, though the tendency to variation might 

 be greatly increased, the regular production of any one variety of 

 young would be prevented. 



Protectional isolation is isolation from the use of different methods 

 of protection against adverse influences in the environment. 



When a new enemy enters the field occupied by any species, 

 different methods of escape or defence are often open to the mem- 

 bers of the one species ; and the use of these different methods must 

 sometimes result in segregating the members according to the methods 

 adopted. Some may hide in thickets or holes, while others preserve 

 themselves by flight. Supposing the species to be an edible butter- 

 fly occupying the open fields and the new enemy to be an insectivorous 

 bird also keeping to the open country, certain members might 

 escape by taking to the woodlands, while others might remain in 

 their old haunt, gaining through protectional selection more and 

 more likeness to some inedible species. 



Nidificational isolation. — Let us now consider the effects of diver- 

 gent habits in regard to nest-building. It is well known to American 

 ornithologists that the cliff swallow of the eastern portions of the 

 United States has for the most part ceased to build nests in the cliffs 

 that were the original haunts of the species, and has availed itself of 

 the protection from the weather offered by the eaves of civilized 

 houses ; and that with this change in nest-building has come a change 

 in some of its other habits. Now, there is reason to believe that if the 

 number of houses had been limited to a hundredth part of those now 

 existing, and if that limited number had been very slowly supplied, 

 this gradual change in some of the elements of the environment 

 would have resulted in divergent forms of adaptation to the environ- 



